Recently, a nonvolatile semiconductor storage device represented by a flash memory has been remarkably developed in capacity. These days, a product having approximately 4 gigabyte capacity is marketed for about several tens of thousands yen. In particular, a mobile or transportable memory such as a USB memory is increasing in commercial value, and its market is growing rapidly so as to take the place of magnetic optical disc and the like.
Moreover, several-gigabyte capacity is large enough for a storage for mobile music player. Apart from a hard disk-mounted mobile music player, which is rapidly spreading, users are attracted to a mobile music player including a nonvolatile semiconductor storage device, which is a solid-state device. This is because such a mobile music player has advantages based on principle of a solid-state memory, such as vibration resistance, high reliability, and low power consumption. Such a nonvolatile semiconductor storage device is expected to become a majority of storages for mobile or transportable products intended for music and image.
Further work on the nonvolatile semiconductor storage device is proceeding for the purpose of realizing a further development in capacity and reduction in bit-cost in view of future possibilities of being a storage for a mobile or transportable product which records/reproduces a moving image.
In particular, with maintenance of advantages of the flash memory, such as low cost and small cell area (not more than 4F2: F indicates the minimum dimension in producing processes), the nonvolatile semiconductor storage device is required to overcome the following problems attributed to operation principle of the flash memory: (i) large writing/erasing voltage (a pressor circuit is required); (ii) slow writing/erasing operation (in particular, erasing requires more than 100 microseconds); and (iii) limited number of rewriting (less than 106times). Overcoming the problems enables the nonvolatile semiconductor storage device to substitute for DRAM, which is used as a main memory in a current information equipment. By this, it becomes possible to realize a so-called “instant-on computer” that starts instantly at the point of use, and achieves next to zero standby power consumption.
As one possibility of such a nonvolatile semiconductor storage device for the next generation, a nonvolatile memory element such as a ferroelectric memory (FeRAM) and a magnetic memory (MRAM), each of which adopts its unique principle, has been studied and developed. However, it is difficult for these elements to be superior to the flash memory in terms of low bit-cost and small cell area.
Under such a circumstance, a phase-change memory (PRAM), a resistance random access memory (RRAM®), and the like are drawing attention as a memory that could be superior in bit-cost to the flash memory. The resistance random access memory herein has, as a data storage section, a variable resistance element including two electrodes and a metal oxide material sandwiched between the two electrodes. It is possible to change an electrical resistance of the variable resistance element by applying, to the two electrodes of the variable resistance element, a voltage (or electric current) that is higher than the threshold voltage (or threshold electric current). Even after once stopping the application of the voltage (or electric current), the resistance state can be maintained in a nonvolatile manner. This enables the resistance random access memory to make data correspond to different resistance states of the variable resistance element so that the data can be stored.
For example, U.S. Pat. No. 6,204,139 (publication date: Mar. 20, 2001; hereinafter referred to as “Patent Document 1”) and “Electric-pulse-induced reversible Resistance change effect in magnetoresistive films” Applied Physics Letter, Vol. 76, pp. 2749-2751, 2000 (hereinafter referred to as “Non-Patent Document 1”) disclose “a method for changing a resistance value by applying electrical pulses having different polarities to thin film made from a perovskite material, which film is sandwiched between a pair of electrodes”. However, the perovskite material has a problem in compatibility with a general semiconductor process.
In order to solve this problem, Japanese Unexamined Patent Publication No. 2004-363604 (Tokukai 2004-363604; publication date: Dec. 24, 2004; hereinafter referred to as “Patent Document 2”) discloses a resistance random access memory including a binary oxide that has a simple composition and a high compatibility with the semiconductor process. Specifically, Patent Document 2 discloses “a nonvolatile memory device having a data storing material layer that exhibits different resistance characteristics for different voltages, wherein a transition metal oxide film having a resistance which rapidly becomes high in response to a voltage within a certain range is NiO, V2O5, ZnO, Nb2O5, TiO2, WO3, or CoO”.
Further, “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses” IEDM Technical Digest, pp. 587-590, 2004 (hereinafter referred to as “Non-Patent Document 2”) describes a nonvolatile resistance random access memory element having an upper and lower electrodes and NiO, TiO2, ZrO2, or HfO2, which is a binary transition metal oxide material sandwiched between the two electrodes.
As described above, the resistance random access memory including the binary transition metal oxide material, which is described in Patent Document 2 or Non-Patent Document 2, has a simple structure/composition and a high compatibility with the semiconductor process, thereby having an advantage of being easily applied to a highly-integrated nonvolatile memory.
However, in the resistance random access memory including the binary transition metal oxide material, which is described in Patent Document 2 or Non-Patent Document 2, a variable resistance element constituting this memory needs to be subjected to such an initializing process called “forming” that causes a soft breakdown. Specifically, the forming intends forming a conductive path, inside the metal oxide material, for electrically connecting two electrodes by generating an electric potential difference of a certain degree between the two electrodes. In case the variable resistance element is applied to a highly-integrated nonvolatile memory, it is extremely important to lower a voltage for the forming (hereinafter referred to as “forming voltage”) in the light of simplification of peripheral circuits.
In this regard, it has been known that the forming voltage is substantially proportional to a film thickness of the metal oxide material sandwiched between the electrodes. It is considered that the forming voltage can be lowered by reducing the film thickness. However, when the film thickness is reduced to such an extent that the forming voltage becomes several volts or less, there arises a problem that a leakage current caused by the reduction in the film thickness increases.